fix: cross-compile Docker image for amd64 and add npm registry auth

- CI docker job: QEMU + buildx + --platform linux/amd64 (runners are arm64)
- Dockerfile: REGISTRY_TOKEN build arg for @cameleer/design-system npm auth
- CI build job: npm auth token for frontend build step
- Registry cache for faster builds

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -1,8 +1,9 @@
 # syntax=docker/dockerfile:1
 FROM node:22-alpine AS frontend
+ARG REGISTRY_TOKEN
 WORKDIR /ui
 COPY ui/package.json ui/package-lock.json ui/.npmrc ./
-RUN npm ci
+RUN echo "//gitea.siegeln.net/api/packages/cameleer/npm/:_authToken=${REGISTRY_TOKEN}" >> .npmrc && npm ci
 COPY ui/ .
 RUN npm run build